1. Field of the Invention
This invention relates to a corrosion resistant protective coating formed on an aluminum substrate. In a particularly preferred embodiment, the invention relates to a high purity protective coating formed on an aluminum substrate by contacting a high purity aluminum oxide coating with one or more fluorine-containing gases to form a coated aluminum substrate capable for use in processing apparatus used to form integrated circuit structures on semi-conductor wafers.
2. Description of the Related Art
The chamber walls of processing apparatus used in the production of integrated circuit structures on semi-conductor wafers such as, for example, chemical vapor deposition (CVD) chambers and/or etching chambers, e.g. reactive ion etching chambers, are subject to attack by the chemicals used in such deposition and etching processes.
In the past, the use of aluminum chambers in semi-conductor wafer processing apparatus with anodized aluminum substrates on the inner walls of the chambers provided sufficient protection against such chemical attack, while permitting the utilization of a relatively inexpensive metal to construct the chamber or chambers of the processing apparatus. However, more recently, the integrated circuit chip industry has recognized the need for yet higher standards of purity in the processing equipment used to fabricate the integrated circuit structures. It has, therefore, been proposed, by Ohmi, in "Fluorine Passivation Technology of Metal Surface", 8th Symposium on ULSI Ultra-clean Technology", The Proceedings, Jan. 26-28, 1989, to replace the anodized aluminum chambers with highly polished stainless steel pretreated in HF to remove oxides, passivated with a high purity F.sub.2 gas to form a non-stoichiometric iron fluoride, and then thermally treated to form an FeF.sub.2 coating. While the resulting film withstands gaseous halogen-containing environments, it will corrode if exposed to an aqueous environment.
It has also been proposed by Ohmi, in "Outgas-Free Corrosion-Resistant Surface Passivation of Stainless Steel for Advanced ULSI Process Equipment", ECS Fall Meeting, Chicago, October, 1988 Symposium of Automated IC Manufacturing, to oxidize passivated highly polished stainless steel materials in O.sub.2 to form a protective oxide surface thereon. Such surfaces are said to be capable of withstanding visible attack by concentrated aqueous hydrochloric acid, i.e., without any visible evidence of evolution of gas, for as long as 30 to 40 minutes.
While a coating with a resistance to corrosion for 30-40 minutes would not normally be considered sufficient for industrial use, it must be pointed out that exposure to aqueous concentrated mineral acids such as hydrochloric acid is considered to be a worst case test, indicative of much longer resistance to corrosion by gaseous halogens.
Therefore, the use of such highly polished stainless steel materials would apparently satisfy the corrosion resistance requirements of the integrated circuit chip industry. However, the cost of the use of such materials in the construction of processing equipment, such as deposition and etching chambers, is prohibitive.
For example, the substitution of an ordinary stainless steel material for aluminum in the construction of an etching or deposition chamber may result in a cost increase of about four times the cost of aluminum, while the use of a highly polished and air oxidized stainless steel may be as much as four times the cost of ordinary stainless steel; i.e., the substitution of such highly polished and specially processed stainless steels for conventional anodized aluminum can result in an increase of costs of over fifteen times what the cost would be to use aluminum.
It would, therefore, be desirable to provide an aluminum material having a corrosion-resistant protective coating on its surface which is capable of resisting the corrosive attack of process halogen gases and plasma (as measured by accelerated corrosion resistance tests using concentrated aqueous halogen acids). It would be even more desirable to provide a high purity corrosion-resistant protective coating which may be utilized on the surface of aluminum parts used in vacuum process chambers so that aluminum may continue to be utilized in the construction of semi-conductor wafer processing equipment for the integrated circuit chip industry without sacrificing purity standards.